As a developing device used in an image forming apparatus such as a copying machine or a printer, there is a device that performs development using a two-component developer. In the developing device that uses the two-component developer, a toner equivalent to an amount consumed by a development operation is supplied. However, in such a developing device, performance of a carrier falls to cause deterioration in charging performance of the toner while the toner is supplied.
A system called trickle development system is provided in order to suppress the deterioration in the charging performance of the toner. The trickle development system is a system for supplying a new carrier (a concentrated toner) to a development container separately from the toner supplied to supplement the consumed amount. An excess developer that cannot be stored in the development container because of the supply of the carrier is discharged from a discharge port. In this way, the deteriorated carrier is replaced with the new carrier little by little.
As such a developing device of the trickle system, for example, JP-B-2-21591 discloses a device that supplies a carrier and a toner to a development container according to development operation and discharges an excess developer from the development container.
On the other hand, in the developing device of the trickle system, the quantity of the developer in the development container changes according to an environmental condition to change the height of the surface of the developer. Therefore, in the developing device, an amount of the excess developer discharged from the discharge port changes according to an environmental change. As a result, an amount of the developer in the development container is not stabilized and, therefore, it is likely that development performance is significantly affected.
Therefore, for example, JP-A-2003-15418 discloses a device that changes the height of a lower surface of a discharge port for an excess developer according to an environmental condition and stabilizes an amount of discharge of the excess developer.
Such a device moves a shutter provided in the discharge port up and down to change the height of the lower surface of the discharge port. This makes it difficult to seal a gap between the discharge port and the shutter to prevent the developer from leaking. It is likely that the developer leaks from the gap between the discharge port and the shutter and soils a section around the discharge port and the shutter. It is also likely that the movement of the shutter is deteriorated by the developer adhering thereto.
Therefore, it is desirable to stabilize an amount of discharge of an excess developer even if an environmental condition changes and, in stabilizing the amount of discharge, prevent the developer from leaking. As a result, there is a demand for the development of a developing device that can suppress a developer amount in a development container from fluctuating, satisfactorily maintain development performance, and maintain a high image quality.